The increased medical use of ultrasound has led to an increased need for devices that can accurately measure the power output of instruments that generate acoustic radiation. Hydrophones and other devices for measuring acoustic power have been available for many years. However to assure accuracy, a power measuring device must be periodically calibrated by determining the response of the device to acoustic radiation of known power. In the past, the calibration of acoustic power measuring devices has been difficult due to the lack of an acoustic source capable of producing acoustic radiation having precise and adjustable peak and average power levels. As a result, most workers in the field have either built nonadjustable power sources for making simple measurements or have used a troublesome reciprocity method to calibrate their sources.